During the processing of silver halide photographic products, silver is removed from the photographic film by contact with a fixing solution. The silver is generally solubilized by reaction with thiosulfate ion. Such a process results in a solution rich in soluble silver.
Two main reasons exist for the recovery of silver in fixing and bleach fixing solutions. First, there exists a regulatory compliance issue. Second, the silver in the solution has monetary value. Another reason for recovering silver, is to reuse a limited resource. In many cases the recovered silver is used again in manufacturing photographic products. Thus, silver recovery is one step in a recycling process.
There are many techniques for recovering silver from photographic solutions: electrolytic silver recovery, metal replacement, ion exchange, chemical reduction, and precipitation methods. Electrolytic silver recovery is one of the most popular methods for the recovery of silver, but is not generally sufficient for those photoprocessors operating in the regulatory compliance mode. Electrolytic techniques do not usually remove silver from photographic solutions to concentrations lower than about 100 ppm. Often a tailing or secondary silver recovery method is also necessary. Ion exchange methods are more suited to secondary silver recovery as are metal replacement cartridges.
Recently, several methods for silver recovery have been disclosed which comprise a precipitation process using a chemical precipitant known as TMT or trimercapto-s-triazine (U.S. Pat. No. 5,288,728, U.S. Pat. No. 5,437,792, U.S. Pat. No. 5,476,593, U.S. Pat. No. 5,496,474.) The precipitation method is a continuous process that replaces a two step silver recovery with a single silver precipitation, flocculation and filtration. The process is simple to use, neat, and consistently allows the photoprocessor to remain in regulatory compliance with respect to silver levels in the photographic effluent. The silver TMT precipitate is easily refined and considered to be a good feed for certain refining operations.
Certain photographic products (mostly films and a few B&W papers) have features that are determined in part by gel hardening that occurs as a result of additives (hardening agents) contained in the fixer solution. The additives are generally high valent cations such as Al.sup.3+ or Zr.sup.4+ that are contained in a low pH fixer solution. Such solutions are sometimes referred to as "hardening fixers." While said additives have a profoundly positive effect for the photographic product, a corresponding difficulty arises for the treatment of the "hardening fixers" during silver recovery using the TMT precipitation process. Since the pH of hardening fixer is usually below 5, the TMT precipitation process is hampered. TMT is a trithiolate compound which undergoes changes via protonation at various pH's and is expected to function most efficiently at pH's above 7 or 8.
A secondary consideration in TMT precipitation is that typical photographic solutions treated by the TMT process contain ammonium ion. The pH of said solutions cannot reasonably be raised above about 8.5 without appreciable production of ammonia gas. Therefore the solutions are generally treated in a range at or below a pH of about 8 to 8.5. If the pH of the solution is below 5, the efficiency of the precipitation of silver by TMT is significantly reduced because at least two of the three active thiolate sites on the molecule are protonated resulting in high levels of silver in the final treated effluent. The decrease in precipitation efficiency at low pH necessitates that a base be added to the silver bearing fixer to raise the pH into a range in which the TMT precipitant functions more effectively.
However, for hardening fixers, a fundamental difficulty arises in any process involving an increase in the pH of the fixer above about 5.5. At such pH's the hardening agent is observed to precipitate. Precipitates of typical hardening agents are exceedingly hydrous and "gel-like." As a result such precipitates are extremely difficult or nearly impossible to filter. Since the TMT precipitation process relies strongly on the facile separation of the silver containing solid from the treated solution, the formation of the a "gel-like" precipitate adversely effects the performance of the process.
The problem is summarized as follows: TMT precipitation is an excellent process for the removal of silver from photoprocessing solutions. However, the pH of hardening fixers must be elevated substantially above 5.5, in order for the TMT to precipitate silver effectively. If the pH of the hardening fixer is raised above 5.5 precipitation of the hardening agent occurs and prevents the efficient operation of the process. The problem to be solved by the present invention is to provide a process for precipitating silver from hardening photoprocessing solutions, without concomitant or subsequent precipitation of the undesired hardening agent.